Siemens 7SA511 manual Voltage Controlled Overcurrent Fault Detection Optional

7SA511 Line Protection Relay Method of Operation

To avoid intermittent pickup signals near the perimeter of the characteristic, a hysteresis of 6% is provided.

Pickup results for the measurement loop in which the impedance vector lies within the fault detection polygon (shaded area in Figure A-3). If detection occurs in more than one loop, the relay regards as valid all loops whose impedance is not greater than 150% of that smallest impedance. This avoids incorrect pickup signals which could be caused by the influence of the fault currents and voltages on the unfaulted line loops, especially in cases of small source impedances.

The impedance fault detection for phase-to-ground loops is supplemented by the overcurrent element (I>>) where pickup of only the overcurrent element leads to detection if the associated impedance loop has been eliminated as described above. In this manner, double faults with a high current are also detected even though the fault loop has been eliminated. Since the overcurrent element can only re-establish eliminated loops for pickup, erroneous fault detection due to an overcurrent is prevented if the short-circuit currents in the fault-free phases can exceed the set overcurrent value on ungrounded feeding transformers or grounded consumer transformers.

Impedance fault detection can be used only for ground faults where the phase-to-ground measurement is enabled by ground fault detection. Phase-phase faults in this case are detected with high-set overcurrent (I>>) element. This detection program is only of advantage in systems with limiters for ground currents (grounded with low impedance— resistance grounding) where a short-circuit current, sufficient for pickup by the overcurrent element, flows for phase-to-phase faults but not for phase-to-ground faults. Ground-free faults are thus detected by the high-set overcurrent element (I>>).

Pickup caused by single-phase ground faults in ungrounded systems is suppressed by the measures described in section A.2.1.

A.2.4 Voltage Controlled Overcurrent Fault Detection (Optional)

Voltage controlled overcurrent (a.k.a. underimpedance) fault detection is a phase-dedicated fault detection procedure which also takes loop information into consideration. The basic characteristic is shown in Figure A-4. Phase pickup occurs when the minimum operating current Iph>, is exceeded. Above this current, voltage controlled overcurrent detection, whose slope is defined by the parameters V(Iph>) and V(Iph>>), is effective. A high-set overcurrent element I>> is superimposed in the case of short-circuits of high current. The bold dots in Figure A-4 identify the setting parameters which determine the geometry of the current/voltage characteristic.

Fault detection of a phase is reset when the respective current is less than 95%, or the respective voltage is greater than 105%. The shaded area in Figure A-4 identifies the fault detection area.

A-12 July 27, 1995